A BAC clone (RP23-35307) carrying the entire mouse Tbx18 gene was recombineered such that the first exon was replaced with the coding sequence for codon-improved cre recombinase (icre) fused to a nuclear localization signial (NLS) and a Myc tag. An frt-flanked Amp(r) selection cassette followed the icre and was removed by transient Flp expression. The cre cassette carries a transcription termination signal so transcription from the Tbx18 promoter is terminated upstream of exon 2, effectively abolishing transcription of any Tbx18 sequence. The linearized construct was microinjected into fertilized oocytes from C57BL/6 x CBA hybrids. Three founders were obtained, crossed to R26R-LacZ reporter mice, and all showed similar reporter gene expression patterns, with line 2 having a lower level of cre activity. Lines 1 and 3 were used for further analysis.